Pharmacological characterization of serotonin synthesis and uptake suggest a false transmitter role for serotonin in the pituitary intermediate lobe

doi:10.1016/0168-0102(94)90156-2

Neuroscience Research

Volume 21, Issue 2, December 1994, Pages 143-149

https://doi.org/10.1016/0168-0102(94)90156-2 Get rights and content

Abstract

A subpopulation of nerve fibers in the rat pituitary intermediate lobe (IL) have been shown to exhibit colocalization of serotonin (5-HT-IR) and tyrosine hydroxylase immunoreactivities and they are sensitive to neurotoxins specific to catecholamine neurons. This study was set out to examine the uptake and synthesis mechanisms of serotonin in these fibers. We developed an in vitro technique in which the neurointermediate lobe explants were incubated (14 and 48 h) in the presence of various drugs and serotonin was subsequently visualized by immunohistochemistry. Control incubation in the presence of serotonin (10⁻⁶ M) resulted in a rich plexus of 5-HT-IR fibers in both posterior and intermediate lobes. Fluoxetine and citalopram (10⁻⁶ M and 10⁻⁵ M), inhibitors of 5-HT transporter, did not affect 5-HT-IR in the IL fibers, unless they were used in concentrations high enough (10⁻⁴ M and 10⁻³ M) to block unspecifically a number of monoamine transporters. The same applied for desipramine (10⁻⁵–10⁻⁷ M), an inhibitor of the noradrenaline transporter. However, cocaine (10⁻⁵–10⁻⁶ M) blocked serotonin uptake into these terminals, suggesting that serotonin uptake occurs through a dopamine transporter. Incubation of the IL in presence of l-tryptophan (10⁻⁴ M) did not result in 5-HT-IR in the IL fibers showing colocalization of 5-HT-IR and tyrosine hydroxylase, which suggests that these fibers do not synthesize serotonin. The present results suggest that serotonin is taken up into the IL terminals by a dopamine transporter and is not synthesized in them, at least in vitro. Thus, serotonin may act as a false transmitter in some of the IL fibers, which may prove to be a valuable model tissue for studying serotonin's role as an exogenously synthesized cotransmitter.

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The mammalian pituitary intermediate lobe: An update on innervation and regulation
2001, Brain Research Bulletin
The pituitary intermediate lobe (IL) in mammals is an area of uniform endocrine cells which synthesize and release specific peptide products of the proopiomelanocortin gene. The lobe receives direct synaptic connections onto the endocrine cells from hypothalamic dopaminergic neurons. This review updates information on the dopaminergic as well as the γ-aminobutyric acid inhibitory neuroregulation for the IL. It also provides a discussion of stimulatory molecules which are likely to affect peptide release, particularly the neurotransmitter serotonin, which may be present via uptake into the dopaminergic nerve terminals. Other stimulatory molecules discussed which are likely to significantly affect peptide secretion are norepinephrine, corticotropin-releasing factor, and several opiate peptides. A new direction of study involves the potential interaction of neurotrophic factors, which are present in all areas of the pituitary, and may be suggested to have a supportive role for the neural elements of the IL. The endocrine cells of the IL and their direct hypothalamic innervation are considered to be an easily accessed peripheral model for study of both neural-endocrine and neurotrophic-target cell interactions.
Inducible expression of tryptophan hydroxylase without serotonin synthesis in hypothalamic dopaminergic neurons
1999, Experimental Neurology
In the present study we have further studied the previous findings that rat hypothalamic dopaminergic neuronal cell groups may express tryptophan hydroxylase (TpH), the serotonin synthesizing enzyme, without a detectable serotonin synthesis. Chemical and mechanical neuronal injuries, namely colchicine treatment and axonal transection, respectively, were performed, and distributions of neurons exhibiting immunoreactivity for TpH and/or tyrosine hydroxylase (TH), the dopamine synthesizing enzyme, were analyzed throughout the hypothalamic periventricular and arcuate nuclei. After colchicine treatment there was a statistically significant 87% (P = 0,01) increase in the number of TpH expressing neurons, while TH expression remained essentially similar. Axonal transection resulted also in a statistically significant 131% (P < 0,01) increase in the number of TpH expressing neurons, while TH expression was not significantly altered. All TpH expression coexisted with TH expression, and the induction of TpH expression by neuronal injuries occurred evenly throughout the rostrocaudal length of the territory studied. A possible serotonin synthesis by TpH was examined by giving drugs that increase brain serotonin synthesis, but no immunohistochemically detectable serotonin synthesis could be found in any of the TpH expressing neurons. Finally the possibility was studied that the relative shortage of the cofactor tetrahydrobiopterin would limit serotonin synthesis. However, an administration of tetrahydrobiopterin did not result in detectable serotonin synthesis in these neurons. Taken together these results suggest that dopaminergic neurons in the hypothalamic periventricular and arcuate nuclei are able to express TpH, this expression is induced after neuronal injury, and this induction occurs similarly throughout the territories studied. TpH expression occurs independently of TH expression, and the newly expressed TpH appears not to synthesize serotonin, regardless of pharmacological pretreatments. Thus, our findings (i) support the idea that neurons may possess inducible expression of nonfunctional transmitter-synthesizing enzymes, in this case TpH, and (ii) suggest that expression of an enzyme synthesizing a certain transmitter may not necessarily imply the corresponding transmitter phenotype.
The ontogenic appearance of tyrosine hydroxylase-, serotonin-, gamma- aminobutyric acid-, calcitonin gene-related peptide-, substance P-, and synaptophysin-immunoreactivity in rat pituitary gland
1998, International Journal of Developmental Neuroscience
The initial appearance of tyrosine hydroxylase (TH)-, serotonin (5-HT)-, gamma-aminobutyric acid (GABA)-, calcitonin gene-related peptide- (CGRP), substance P-, and synaptophysin-immunoreactivity in the rat pituitary gland, and in the related brain regions was investigated. Several groups of TH-immunoreactive neurons were first detected in the brain stem on day E17, and in the hypothalamus on day E18, followed by TH-immunoreactivity in the median eminence and infundibulum on E19–E20. TH-positive fibers appeared in the posterior lobe on day E20 and in the intermediate lobe on day P0. 5-HT-immunoreactivity was first detected on day E17 in neurons and nerve fibers in the brain stem and in the median eminence, respectively. On day E18, a few 5-HT-immunoreactive fibers were detected in the posterior lobe of the pituitary, although they were consistently seen in the infundibulum from day E19. In newborn rats, some 5-HT-immunoreactive fibers, but no neurons, were seen in the hypothalamus. GABA immunoreactivity appeared on day E17 in several nerve fibers of the infundibulum and the posterior lobe. Some neurons in the cortex and ventral hypothalamus transiently expressed GABA immunoreactivity on day E17. In newborn rats, a plexus of GABA-immunoreactive fibers was detected for the first time in the intermediate lobe. No CGRP-immunoreactive fibers could be detected in the prenatal pituitary. On day P10, CGRP-immunoreactive fibers were first observed in the anterior lobe. Later their number considerably increased, while only sporadic fibers could be found in the intermediate or posterior lobes. No substance P-immunoreactivity could be detected in any of the lobes in the embryonic or developing postnatal rat pituitary, instead the adult anterior lobe occasionally showed some substance P-immunoreactive fibers. Synaptophysin-immunoreactivity was first detected in the posterior lobe on day E20, followed shortly by its expression in the intermediate lobe in newborn rats. The time course of GABA and 5-HT expression revealed in the present study suggests that these transmitters, which are initially expressed in the developing pituitary clearly before synaptic maturation, may act as trophic molecules during the prenatal period.
Chronic inhibition of the high-affinity dopamine uptake system increases oxidative damage to proteins in the aged rat substantia nigra
1997, Free Radical Biology and Medicine
The effect of chronic treatment of aged rats with nomifensine has been studied in the rat nigrostriatal dopaminergic system. The rat substantia nigra suffers an oxidative damage during aging that results in both an increase in carbonyl groups of its total proteins and the oxidative inactivation of tyrosine hydroxylase (TH) enzyme,[1] which are partially reversed by chronic treatment with deprenyl. Different mechanisms may account for this effect, including inhibition of the high-affinity dopamine uptake system. We treated aged rats chronically with nomifensine for 2 months and found some significant effects. Nomifensine treatment significantly increased TH enzyme amount in substantia nigra (39.2%), which was accompanied by a significant increase in TH enzyme activity (47.8%). However, these effects were not observed in the terminal field (striatum). As a further step we quantified the oxidative level of proteins by measuring the number of carbonyl groups coupled either to total proteins or specifically to TH enzyme. The proteins of aged rat substantia nigra showed a significant increase of carbonyl groups following nomifensine treatment. The number of carbonyl groups coupled to nigral TH enzyme also increased in the nomifensine-treated animals. However, this increase was lower than that found in the total homogenate proteins. All these results show that the oxidative damage produced during aging in tyrosine hydroxylase enzyme and total proteins is not reduced by nomifensine treatment. On the contrary, the nomifensine treatment increased the oxidative damage to proteins. These results suggest the capability of deprenyl to induce TH enzyme could be due to inhibition of the high-affinity dopamine uptake system, but its ability to protect against oxidative damage is not produced by this mechanism. © 1997 Elsevier Science Inc.
Dopaminergic periventriculo-hypophyseal nerves show tryptophan-hydroxylase immunoreactivity but lack serotonin synthesis
1995, Brain Research
Hypothalamic dopaminergic periventricular and arcuate nuclei are known to the project to the pituitary gland and contain serotonin in their terminals. In order to elucidate the potential of these neurons to synthesize serotonin, we studied immunohistochemically the possible trypthophan hydroxylase content of periventriculo-hypophyseal neurons, identified by retrograde tracing from the pituitary gland. These neurons were found to contain tryptophan hydroxylase-immunoreactivity (TpOH-IR), which was enhanced after colchicine treatment. All of the TpOH-IR neurons contained tyrosine hydroxylase-immunoreactivity as well. However, none of them were immunoreactive for serotonin in either intact animals or in animals pretreated with serotonin precursor l-tryptophan and MAO inhibitor pargyline. Thus, neurons of the dopaminergic periventriculo-hypophyseal pathway express tryptophan hydroxylase, but are unable to synthesize serotonin. These findings (i) raise the possibility that, in these nerves, serotonin might serve a function other than regular synaptic transmission, and (ii) suggest that expression of an enzyme synthesizing certain transmitter does not necessarily confirm the corresponding transmitter phenotype of that neuron.
Release of false transmitter serotonin from the dopaminergic nerve terminals of the rat pituitary intermediate lobe
1995, Neuroscience Research
Rat pituitary intermediate lobe contains two types of serotonin-immunoreactive nerve terminals. Most of them are dopaminergic, in which serotonin acts as a false transmitter, while the rest are true serotonergic nerves. In the present study, release of the false transmitter serotonin from the dopaminergic nerve terminals was studied by loading the neurons in vivo with serotonin precursor l-tryptophan and MAO inhibitor pargyline, which results in accumulation of false transmitter serotonin. Subsequently pituitary neurointermediate lobe complexes were incubated in the presence of various agents. Potassium induced dramatic release of serotonin. This release was Ca²⁺-dependent, as demonstrated by an inhibition by Mg²⁺, and transporter-independent, since it was unaffected by GBR 12909 (a dopamine transport inhibitor). Tyramine and sodium nitroprusside, a nitric oxide donor, caused slight to remarkable release of serotonin. This release was inhibited by GBR 12909, suggesting that it was transporter-dependent. Presynaptic stimulation with apomorphine or haloperidol, dopamine receptor agonist or antagonist, respectively, or isoproterenol, agonist of the β-adrenergic receptor, did not significantly release serotonin. Thus, it seems that presynaptic receptors per se cannot induce release of significant amounts of serotonin from the IL dopaminergic fibers. Our results suggest that false transmitter serotonin in the IL dopaminergic nerve terminals is released primarily by the classical exocytotic release mechanism, but may also be partly released by the transporter-dependent, non-exocytotic release.

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Research reportPharmacological characterization of serotonin synthesis and uptake suggest a false transmitter role for serotonin in the pituitary intermediate lobe

Abstract

Catecholamine-synthesizing enzymes in the rat pituitary. An immunohistochemical study

Histochemistry

Cloning and expression of a functional serotonin transporter from rat brain

Nature

In vivo effects of serotonergic agents on alpha-melanocyte-stimulating hormone secretion

Neuroendocrinology

Platelet-induced neurogenic coronary contractions due to accumulation of the false neurotransmitter, 5-hydroxytryptamine

J. Clin. Invest.

Inhibition of adrenergic neurotransmission in canine tibial artery after exposure to 5-hydroxytryptamine in vitro

J. Pharmacol. Exp. Ther.

Platelet 5-Hydroxytryptamine and vascular adrenergic nerves

News Physiol. Sci.

Serotonergic innervation of the rat pituitary intermediate lobe: decrease after stalk section

Endocrinology

Evidence that hypothalamic periventricular dopamine neurons innervate the intermediate lobe of the rat pituitary

Neuroendocrinology

Cloning of a serotonin transporter affected by antidepressants

Science

The dopaminergic innervation of the intermediate lobe and of the neural lobe of the pituitary gland

Med. Biol.

Functional topography of the rat hypothalamic dopamine neuron systems: retrograde tracing and immunohistochemical study

J. Comp. Neurol.

Cloning and expression of a cocaine-sensitive rat dopamine transporter

Science

Le lobe intermédiare de l'hypophyse, modéle de communication neuroendocrinienne

Arch. Int. Phys. Biochim. Biophys.

Serotonin immunoreactive nerve fibers and terminals in the rat pituitary-light- and electron-microscopic studies

Neuroscience

Uptake of 5-hydroxytryptamine and 5-hydroxytryptophan by neurons of the central nervous system normally containing catecholamines

J. Neurochem.

On the origin of the serotonergic input to the intermediate lobe of the rat pituitary

Brain Res.

Citalopram. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic potential in depressive illness

Drugs

Expression cloning of a cocaine- and antidepressant-sensitive human noradrenaline transporter

Nature

Serotonin-containing elements of the rat pituitary intermediate lobe

Neuroendocrinology

Serotonergic elements of the mammalian pituitary

Endocrinology

Research report
Pharmacological characterization of serotonin synthesis and uptake suggest a false transmitter role for serotonin in the pituitary intermediate lobe